When drilling a well bore, it is desirable for the pressure of the drilling fluid in the newly drilled well bore, where there is no casing, to be greater than the local pore pressure of the formation to avoid flow from, or collapse of, the well wall. Similarly, the pressure of the drilling fluid should be less than the fracture pressure of the well to avoid well fracture or excessive loss of drilling fluid into the formation. In conventional onshore (or shallow offshore) drilling applications, the density of the drilling fluid is selected to ensure that the pressure of the drilling fluid is between the local formation pore pressure and the fracture pressure limits over a wide range of depths. (The pressure of the drilling fluid largely comprises the hydrostatic pressure of the well bore fluid with an additional component due to the pumping and resultant flow of the fluid.) However, in deep sea drilling applications the pressure of the formation at the seabed SB is substantially the same as the hydrostatic pressure HP of the sea at the seabed and the subsequent rate of pressure increase with depth d is different from that in the sea, as shown in FIG. 1a (in which P represents pressure and FM and FC denote formation pressure and fracture pressure respectively). This change in pressure gradient makes it difficult to ensure that the pressure of the drilling fluid is between the formation and fracture pressures over a range of depths, because a single density SD drilling fluid does not exhibit this same step change in the pressure gradient.
To overcome this difficulty, shorter sections of a well are currently drilled before the well wall is secured with a casing. Once a casing section is in place, the density of the drilling fluid may be altered to better suit the pore pressure of the next formation section to be drilled. This process is continued until the desired depth is reached. However, the depths of successive sections are severely limited by the different pressure gradients, as shown by the single density SD curve in FIG. 1a, and the time and cost to drill to a certain depth are significantly increased.
In view of these difficulties, dual density DD drilling fluid systems have been proposed (see US2006/0070772 and WO2004/033845 for example). Typically, in these proposed systems, the density of the drilling fluid returning from the wellbore is adjusted at or near the seabed to approximately match the density of the seawater. This is achieved by pumping to the seabed a second fluid with a different density and mixing this fluid with the drilling fluid returning to the surface. FIG. 1b shows an example of such a system in which a first density fluid 1 is pumped down a tubular 6 and through a drilling head 8. The first density fluid 1 and any cuttings from the drilling process then flow between the well wall and the tubular. Once this fluid reaches the seabed, it is mixed with a second density fluid 2, which is pumped from the surface SF via pipe 10. This mixing process results in a third density fluid 3, which flows to the surface within a riser 4, but is also outside the tubular 6. The fluids and any drilling cuttings are then separated at the surface and the first and second density fluids are reformed for use in the process.
In alternative proposed systems, a single mixture is pumped down the tubular and when returning to the surface the mixture is separated into its constituent parts at the seabed. These separate components are then returned to the surface via the riser 4 and pipe 10, where the mixture is reformed for use in the process.
With either of the dual density arrangements, the density of the drilling fluid below the seabed is substantially at the same density as the fluid within the tubular and the density of the first and second density fluids may be selected so that the pressure of the drilling fluid outside the tubular and within the exposed well bore is between the formation and fracture pressures.
Such systems are desirable because they recreate the step change in the hydrostatic pressure gradient so that the pressure gradient of the drilling fluid below the seabed may more closely follow the formation and fracture pressures over a wider range of depths (as shown by the dual density DD curve in FIG. 1a). Therefore, with a dual density system, greater depths may be drilled before having to case the exposed well bore or adjust the density of the drilling fluid and significant savings may be made. Furthermore, dual density systems potentially allow deeper depths to be reached and hence greater reserves may be exploited.
However, one problem with the proposed dual density systems is that when the flow of drilling fluid stops, there is an inherent hydrostatic pressure imbalance between the fluid in the tubular and the fluid outside the tubular, because the fluid within the tubular is a single density fluid which has a different hydrostatic head to the dual density fluid outside the tubular. There is therefore a tendency for the denser drilling fluid in the tubular to redress this imbalance by displacing the less dense fluid outside the tubular, in the same manner as a U-tube manometer. The same problem also applies when lowering casing sections into the well bore.
UK patent application (GB0802856.5) addresses this issue by providing a flow stop valve positioned in a downhole tubular. (GB0802856.5 is herein incorporated by reference.) The flow stop valve described therein is in a closed position when a pressure difference between fluid outside the downhole tubular and inside the downhole tubular is below a threshold value, thereby preventing flow through the downhole tubular. Furthermore, the flow stop valve is in an open position when the pressure difference between fluid outside the downhole tubular and inside the downhole tubular is above a threshold value, thereby permitting flow through the downhole tubular. The flow stop valve described in GB0802856.5 is therefore opened by a “cracking” pressure provided by pumps and the flow stop valve is otherwise closed to prevent the flow of fluid due to the imbalance in hydrostatic pressures.
However, in some embodiments of such a valve, the valve may chatter when it is opened because once the flow stop valve has opened, the localised pressure above the valve reduces, thereby tending to close the valve again. The present invention therefore seeks to address this issue.